It sounds like your son may be a young or school-aged child as this kind of infection is very common in this age group (although it can also affect adults). Overall gastroenteritis is a generic term used to refer to inflammation and irritation of the gastrointestinal tract (including the stomach and intestines), and you’ll want to check with your pediatrician
or primary care doctor
about his illness
Gastroenteritis is typically caused by one of many different viruses and can cause abdominal discomfort or cramping, nausea or vomiting, and sometimes diarrhea. Hopefully your son isn't feeling too poorly! The good news about viral gastroenteritis is that although the symptoms may be unpleasant, the infection is typically self-limiting and will go away on its own. There are no specific treatments for these kinds of viral infections, so the most important thing you can do is make sure that he stays hydrated. Someone with viral gastroenteritis may not feel like eating much, but it would be important to intake enough fluids--at least a few sips every couple minutes, averaged over the day--to avoid becoming dehydrated.
Any questions about your son's illness, diagnosis, and treatment can also be addressed by his physician, either pediatrician or primary care doctor. Hopefully he'll be feeling better in no time!